The prior art is replete with electro-acoustic transducers, particularly usable for underwater acoustic detection and transmission. Desirable properties of such transducers are: a high hydrostatic piezoelectric coefficient (d.sub.h) and a high hydrostatic voltage coefficient (g.sub.h); a relatively high dielectric constant; a hydrostatic sensitivity in the low frequency range; and no variation of g.sub.h with changing hydrostatic pressures.
The hydrostatic piezoelectric coefficient d.sub.h is given by the equation: d.sub.h =d.sub.31 +d.sub.32 +d.sub.33. d.sub.33 denotes the uniaxial piezoelectric coefficient for the relationship of polarization in the "3" direction (thickness dimension) to stress in that direction. d.sub.31 and d.sub.32 are uniaxial piezoelectric coefficients for orthogonal directions in the transverse plane.
Piezoelectric ceramic materials, such as lead zirconate titanate, are often used in acoustic transducers and, if submerged in a liquid, see constant and equal pressures applied to all sides of the transducer. The piezoelectric coefficient d.sub.h, under water, is very small because d.sub.33 and the d.sub.31, d.sub.32 values are opposite in sign and almost cancel one another.
The prior art has recognized that one or more of the uniaxial piezoelectric coefficients must be altered in order to maximize the hydrostatic piezoelectric coefficient. For instance, in U.S. Pat. No. 4,649,312 to Robin, et al., the d.sub.31 and d.sub.32 uniaxial piezoelectric coefficients are minimized by forming a grid of fibers which are interwoven and then overmolded with a piezoelectric material. This results in the grid and its encompassing piezoelectric material. This results in the grid and its encompassing piezoelectric forming an integral structure, which when subjected to pressure, enables the piezoelectric effects due to the compressive forces normal to the plane of the structure, to predominate.
Others have attempted to improve a hydrophone's uniaxial piezoelectric coefficient d.sub.31 by combining piezoelectric polymer material and conductive polymer with a metal sheet as an electrode through the use of piezoelectric material, (U.S. Pat. No. 4,786,837 to Kalnin, et al.).
An object of this invention is to provide an improved piezoelectric ceramic based transducer, wherein the d.sub.31 and d.sub.32 piezoelectric coefficients augment the d.sub.33 coefficient rather than detracting from it. This is accomplished by inserting a cavity in the metal electrode. The cavity transforms the incident pressure wave to an internal radial stress on the ceramic, thereby enhancing the electrical response of the transducer.
A further problem with piezoelectric-based acoustic transducers is the aging effect on the polarized piezoelectric ceramic. As is known, piezoelectric ceramics may be poled by applying a high electric field across the sample at an elevated temperature and subsequently cooling the piezoelectric ceramic to room temperature. Subsequently, a certain percentage of the aligned dipoles is observed to randomly reorient ("age"), thereby reducing the effectiveness of the ceramic's piezoelectricity.
It is another object of this invention to provide an improved piezoelectric transducer wherein aging is minimized and strength is provided to withstand high hydrostatic pressure.